1. Field of the Invention
The present invention relates to a light beam deflecting apparatus (hereinafter a light beam deflecting device, a drive unit and a control unit therefor being collectively called a light beam deflecting apparatus or a light beam deflecting unit), and an image forming apparatus, and more particularly to a light beam deflecting unit and an image forming apparatus such as a laser display, a laser printer or a scanner.
2. Related Background Art
There is conventionally known a light beam deflecting scanning apparatus for deflecting a light beam, emitted from a light source such as a laser, by a light beam deflecting device such as a polygon mirror or a galvanometer to scan a screen or an exposure surface. The polygon mirror is a deflecting device in which plural mirrors constituting a polygon are rotated by a periodical drive signal to effect light beam scanning in one direction on the screen. The gavanometer causes a vibrational reciprocating motion to a mirror surface under the application of a periodical drive signal to generate reciprocating light beam scanning on the screen. It is naturally possible also to utilize the scanning of one direction only by partially turning off the light beam.
For driving the galvanometer, there is known electrostatic drive, electromagnetic drive or drive utilizing piezo effect. The wave form of vibration is generally sinusoidal, but vibration of triangular wave or saw tooth wave is also possible by suitably selecting the wave form of the driving voltage. In case the galvanometer has an intrinsic vibration, a drive signal of a matching frequency can be given to generate resonance thereby causing the vibration of maximum amplitude with a minimum electric power.
There is proposed a projection image display apparatus utilizing such light beam detecting device for projecting an image such as of television onto a screen. In such image forming apparatus, there is known an apparatus provided with a light beam detecting device for executing horizontal and vertical scans with a laser light beam emitted from a laser light source such as LD, by resonant drive of two galvano mirrors.
In such image display apparatus utilizing the light beam deflecting device, there is generally involved a difference between the periodical drive signal applied to the deflecting device and the actual rotational or reciprocating rotational movement in the deflecting device or the periodical scanning motion of the light spot on the screen, as will be explained in the following.
FIGS. 10A to 10E shows the relationship in time of the drive voltage applied to the galvanometer and the position of the actual light spot on the screen. FIG. 10A shows the change in time of the drive voltage applied to the galvanometer, and FIGS. 10B to 10E show the changes in time of the position of the light spot on the screen. FIG. 10B shows a standard response, while FIG. 10C shows a case where the phase is shifted, FIG. 10D shows a case where the amplitude is changed, and FIG. 10E shows a case where the center position is shifted. In practice, these states may appear in mixed manner.
Such shift in the phase, amplitude or central position principally results from a fluctuation in the characteristics of the galvanometer, a variation in the temperature, a variation in the position of the mirror and the light source, and a variation in the position of the mirror and the screen.
The vibration characteristics of the galvanometer vary depending on the dimension of the mirror and a torsion shaft supporting such mirror, and the distance between the drive electrodes or between the electromagnetic coils, but certain fluctuations in these factors are unavoidable in the manufacture, so that each galvanometer has delicate difference in the characteristics. For this reason, the response is different for the same voltage, whereby the maximum displacement angle, namely the amplitude and the phase, may shows a shift from the reference value.
In case of resonant vibration, a change in the temperature causes a change in the intrinsic vibration, which is therefore shifted from the frequency of the drive signal. The amplitude and the phase are shifted also by such factor. FIGS. 11A and 11B schematically show such phenomenon, wherein FIG. 11A shows the relationship between the drive frequency and the amplitude while FIG. 11B shows the relationship between the drive frequency and the phase. The maximum amplitude can be obtained if the drive frequency is at a state A matching the resonant point, but, if the resonance frequency is shifted to a state B, the amplitude decreases with a phase shift. The phase shift occurs in opposite directions depending on the direction of displacement, and the phase is delayed in case the resonance frequency changes to the higher side while it proceeds in case the resonance frequency changes to the lower side. If the relative position of the light source with respect to the screen changes in addition to the fluctuation in the characteristics of each mirror and the variation in temperature, the incident angle of the light beam into the mirror changes, whereby the center position of scanning on the screen is shifted. The center position is further shifted if the set position of the screen is shifted.
The foregoing explanation is based on a case of sinusoidal vibration, but the situation is similar also in other vibrations. In contrast, in case the deflecting device is constituted by a polygon mirror, the scanning amplitude does not vary in principle because the maximum displacement angle is determined by the number of the mirror faces, and the aforementioned change in the amplitude and phase by the temperature does not occur since the resonance is not utilized.
On the other hand, the light beam emitted from the light source is subjected to intensity modulation by the image signal and displays an image on the screen, and the start and end timings of such modulation are synchronized with the mirror drive signal in order to display the image in a predetermined area on the screen. However, if the relationship between the drive signal and the light spot on the screen is shifted from the reference state, there cannot be properly obtained a predetermined image on the screen. For example, if the light spot is caused to scan with a delay from the predetermined timing because of a phase shift as shown in FIG. 10C, the image is displayed in the upstream side of the predetermined position in the scanning. In case of one-dimensional scanning as in the polygon mirror, the image is merely displaced in the lateral direction and such displacement is permissible if it is slight, but, in case of reciprocating scanning, the image is shifted in opposite directions in the forward and backward scanning motions to generate doubled images, thus causing a fatal defect. Also a change in the amplitude or a change in the center position may cause, depending on the level thereof, a significant defect in the image.
In order to avoid such defect, it is necessary, in the actual display apparatus, to monitor the mode of vibration of the galvanometer or the movement of the light spot on the screen and to adjust the drive signal for the deflecting device or the modulation timing of the light beam. The following monitor methods have been proposed.
A galvanomirror disclosed in the Japanese Patent No. 2657769 is provided with a permanent magnet to superpose a high frequency wave with the drive current, and the displacement angle of the galvanomirror is detected from the change in the mutual inductance of between a drive coil on the galvanomirror and a detection coil opposed to the galvanomirror.
Also the Japanese Patent No. 3011144 discloses a method of measuring the rocking position of the galvanomirror by superposing an AC voltage of a small voltage and a high frequency with the drive voltage and detecting the current difference flowing in an electrode fixed to the electrode portion of the galvanomirror having a capacitor structure.
Also the Japanese Patent Application Laid-open No. 64-28535 discloses a method of measuring the rotational fluctuation or jitter of the mirror, from the difference in time of the detections in two positions of the light reflected from the polygon mirror.
The displacement angle sensor described in the Japanese Patent Nos. 2657769 and 3011144 has the advantage of being incorporated in the deflecting device and being capable of detecting the displacement of the galvanomirror over the entire period, but is therefore difficult to detect a small shift in the amplitude or in the phase. The detector is required to have a time resolution sufficient for determining the modulation start timing, but the change in the coil inductance or in the displacement current is very weak in the deflecting device executing a very small vibration, and the highly precise determination of the displacement angle is practically difficult, involving complication of the apparatus and an increased cost thereof.
On the other hand, the method disclosed in the Japanese Patent Application Laid-open No. 64-28535 is based on the detection of the light reflected by the polygon mirror and has a high precision of time measurement, but is associated with the following drawbacks in application to the sinusoidal vibration of the galvanomirror.
A sinusoidal vibration is uniquely defined by three parameters of amplitude A, phase δ and center position X0 (period T being constant and defined as known). The change in time of the spot position X on the screen can be represented, with these parameters, by:X=A sin(2πt/T−δ)+X0.These parameters vary by various factors as explained in the foregoing, but the spot position on the screen at an arbitrary time can be given if all these parameters are known.
If the parameters are determined and the foregoing equation is established, and in case of displaying an image within a range XA<X<XB wherein XA and XB are end positions of the image on the screen, there can be determined timings tA and tB respectively corresponding to XA and XB from the equations:XA=A sin(2πtA/T−δ)+X0 andXB=A sin(2πtB/T−δ)+X0and the modulation signal to be supplied to the light source can be so controlled as to initiate the modulation of the image signal at tA and to terminate the modulation at tB. As the reference of tA and tB, the timing can be suitably selected for a fixed signal having a period of vibration of the galvanomirror, but, in the following, the reference is taken at the rising edge of a signal synchronized with the drive signal for the galvanomirror.
In this manner, by automatically calculating the timing of modulation and executing the actual modulation based on such calculation, the image can always be displayed with a predetermined size and in a predetermined area even in the presence of any fluctuation in the characteristics of the deflecting device or of any shift in the positional relationship between the light source and the screen.
Then, let us consider the determination of the aforementioned three parameters.
At first, by obtaining measurement data:X1=A sin(2πt1/T−δ)+X0X2=A sin(2πt2/T−δ)+X0by detecting the light in two positions on the screen, it will be evident that three unknown parameters A, δ and X0 cannot be determined, so that it is necessary to detect the light in three or more positions.
Also, even if the third measurement data:X3=A sin(2πt3/T−δ)+X0by detecting the light in three positions, it is still not easy to determine the three unknown parameters A, δ and X0 from these measurement data, because δ is included as a non-linear parameter. Such determination involves complex calculations and requires a calculation time. Particularly in case the aforementioned detection is executed in every scanning line, there is required high speed calculation not exceeding a line scanning time, and it is difficult to meet such requirement with a simple circuit.
Among the three parameters, X0 can be easily adjusted by moving the position of the deflecting unit or the screen under the observation of an image projected by the light beam on the screen. In such case, the number of the unknown parameters is reduced to two, but the detector is still required in two positions and the complexity of calculation in determining δ still remains.
Also by calculating the difference of the aforementioned two data, there is obtained:
 X1−X2=A{sin(2πt1/T−δ)−sin(2πt2/T−δ)}
which no longer contains X0 but still contains two parameters A, δ, so that the amplitude cannot be uniquely determined from the difference data.
On the other hand, in case of a polygon mirror, there can be obtained a relationship:X=A(t/T−δ)+X0since the change in time of the spot position on the screen is linear after correction with an fθ lens. Though this relation still contains three parameters, namely amplitude A, phase δ and center position X0, but the number of parameters is substantially two because the two parameters δ and X0 are expressed in a form −Aδ+X0. This is based on a situation that the change in phase and the change in center position appear as a shift in the image position and cannot be separated on the screen. Consequently, in such case, the unknown parameters can be eliminated from the measurement data in two positions:X1=A(t1/T−δ)+X0X2=A(t2/T−δ)+X0and the positional change on the screen can be completely reproduced. More specifically, at first there is obtained the difference data:X1−X1=(A/T)(t1−t2)from which A/T is determined. Then it is substituted in the equation of T1 which is solved for −Aδ+X0. Then this is substituted in the equation of X to obtain:X=(X1−X2)/(t1−t2)×(t−t1)+X1
As explained in the foregoing, since the galvanomirror is executing reciprocating motion, the timing detection method for the scanning light cannot be applied to the galvanomirror unlike the case of the conventional polygon mirror executing the one-directional scanning with a constant speed. Also since the scanning speed of the galvanomirror varies, the displacement angle is difficult to estimate except for the timing of detection, by mere detection of the timing of scanning. In case of employing a galvanomirror in a laser display or the like, the laser beam intensity is modulated by the image information while the light is put into the reciprocating scanning motion by the galvanomirror, but the estimation of the displacement angle at an arbitrary time is necessary in order to generate the start/end timing of modulation.
The object of the present invention is to resolve an issue that, in an image display apparatus employing an optical deflecting device executing non-linear scanning such as sinusoidal scanning, in case the relationship between the drive signal and the actual motion of the deflecting device or the relationship between the drive signal and the light beam scanning on the actual screen is shifted from a predetermined state by any reason, the conventional method of automatically detecting the motion of the deflecting device requires an increased number of detectors and there is required a complex and difficult calculation for determining the unknown parameters from the obtained data.